Automatic displacement elements are used in a wide variety of applications to enable the user to operate windows, doors and other closing devices conveniently and easily. For this purpose a DC motor is generally used which serves to drive the displacement element via an appropriate mechanism. Particularly in the automotive field, automatic window lifts and sunroofs as well as doors and lift gates have become commonplace and are already being fitted as standard in the vast majority of new cars.
However, particularly in the automotive field, the potential hazard of electric window lifts or sunroofs is already well recognized also, as numerous accidents involving them have already been recorded and given extensive media coverage.
If, for example, an object or body part is trapped between vehicle frame and electrically operated window or door, it may be subject to adverse effects or crushing because of the not inconsiderable motive force of the power actuator. Children, dogs or even adults can be injured by accidental actuation of electrical closing devices, such accidents even proving fatal in extreme cases. This is particularly the case with an automatic up/down feature whereby a brief touch of the switch suffices to open or close a window automatically. Tests have found that a closing force of even 100 Nm acting on the human neck can be life-threatening, and for small children the danger limit is as low as 30 Nm.
The experience of recent decades has therefore made it necessary to create devices for limiting the closing force, i.e. providing anti-trap protection, in order to stop the closing movement of motor-driven elements such as windows and doors if necessary and prevent such accidents from occurring. Although an EU directive specifies limiting the force of window lifts to 100 Nm, many automobile manufacturers are voluntarily aiming for a limitation to 10 Nm in order to make sure that any kind of crushing of fingers or other extremities is eliminated.
For this purpose any resistance to the closing movement of the window is registered by an open-loop controller, causing the motor-driven closing movement to be stopped or, in the case of intelligent control, even reversed by reversing the motor direction so that the trapped object or body part is immediately released. The window or door closing movement therefore lasts only until such time as an obstacle gets in the way.
A resistance is mainly detected by means of computer-assisted analysis of the motor current. If an object impedes the movement of the window pane, the motor is slowed down and the motor current increases. In such cases the controller which is measuring the motor current interrupts the supply of current or causes the displacement element, i.e. the window pane, to return in the opposite direction by reversing the polarity at the terminals of the DC motor via a suitable circuit.
Particular requirements are also placed on the software components of closed-loop control units, as the control system must not only detect a real obstacle, but also differentiate it from a defect such as icing-up or sticking of the power window due to dirt. Since in the latter case the resistance occurring must be overcome and the control unit must not be “fooled” by changing operating and environmental conditions, precisely operating and intelligent software solutions are required.
For driving window lifts, sunroofs, lift gates and other movable elements (hereinafter referred to globally as displacement elements), DC motors are normally used, whereby there is mounted on a motor shaft an at least two-pole rotor by means of whose rotation the motor's rotary motion is converted via Hall sensors into a Hall signal which is in turn used for speed calculation. The Hall sensor is a semiconductor device which produces a voltage as the result of current flow and an external magnetic field, said voltage increasing with the intensity of the current flow and the magnetic flux density.
As the Hall sensor changes its voltage level more quickly the faster the motor shaft rotates, the speed of the displacement element during its translational opening or closing movement can be determined, the motor speed being dependent on the voltage present and the necessary force which the motor must exert to produce the desired movement of the displacement element. Due to changing operating conditions such as temperature, gearing and various frictional resistances caused in particular by rubber seals, the force required to move the displacement element varies, which causes the speed of the system to vary while the voltage dropped across the motor remains constant.
As it is desired on the part of the industry to keep the displacement element speed constant throughout the opening and closing movement, the voltage applied to the motor is varied accordingly. In practical terms this means that the voltage must be increased equivalently the more force the motor requires to maintain the desired speed even under changed conditions. For this purpose the system is clocked via pulse width modulation (PWM), the input voltage supplying the motor being switched on and off at a high frequency of normally 20 kHz in short variable cycles. These cycles are termed the switching period Ts, the ratio of the on-time ton to the off-time toff during such a switching period Ts being variable as required.
If the on-time ton is increased, a larger arithmetic mean of the output voltage and therefore a higher output current is produced. In technical terms this is also known as a “duty cycle”, whereby if the on-time ton and off-time toff are of equal length a duty cycle of 50% is present, which means that the input voltage is also halved. If the on-time ton is only a quarter of the switching period Ts, this is termed a duty cycle of 25% with consequently only a quarter of the input voltage being applied to the motor. The duty cycle and therefore the output of the motor is continuously controllable from 0 to 100%.
In known methods according to the prior art, the required displacement force which the motor needs to move the displacement element is calculated via the voltage and speed of the motor and stored in a nonvolatile memory. For this purpose a learning run is executed subject to clocking by means of constant PWM in order to determine the various frictional forces occurring in the system over the movement range of the displacement element. The frictional forces result primarily from the contact of the displacement element with seals and other mechanical transitions.
A learning run is necessary for each individual closing system because, in spite of standardization and mass production, every mechanical system proves to be unique and possesses individual characteristics which means that, due to manufacturing tolerances in the mechanism, it also does not behave in the same way in terms of its movement. Thus prior to initial commissioning of a new system, a one-off learning run is therefore performed and the characteristic data obtained is stored as a frictional or displacement force curve in order to then serve as a reference for all subsequent closing movements of the displacement element during normal operation.
For all the closing movements taking place in the future, the required PWM clocking is determined by means of complicated calculations on the basis of motor voltages and the associated stored reference data concerning the displacement forces which is obtained during the learning run in order to enable the different mechanical forces present at various points in time to be compensated. Comparison of the reference data with the forces currently present during a closing movement of the displacement element finally allows an object or body part to be detected and suitable control pulses to be triggered in order to stop the closing movement or reverse it by reversing the polarity of the direction of the motor. In practical terms, the exceeding of a particular permissible displacement force is therefore computationally registered and the motor drive is controlled accordingly in order to release the trapped object, the duty cycles for controlling the motor with constant speed being calculated on the basis of the characteristic values obtained from the learning run according to the displacement forces and not on the basis of the current speed actually occurring in the system.
As the learning run merely constitutes a simulation of the movement sequence of the displacement element as it occurs under learning run conditions (in the laboratory, workshop, etc.), but cannot allow for any current operating conditions and environmental effects present at the time of any displacement movements occurring subsequent to the learning run under real environmental conditions, it is also realistically impossible to match the speed to new circumstances.
One of the disadvantages of this method is that the reference data obtained during the learning run is also used as reference for all movements of the displacement element taking place in the future and the speed of the system is always reproduced in a rigid manner according to the calculation performed for the learning run. As the system's mechanism is subject to aging and changing environmental and operating conditions such as increased dust and temperature exposure, the speed of the displacement element cannot be kept constant and tends to vary from one path section to another, which also makes it difficult to define a precise closing force limitation, with the result that extremities trapped by the displacement element may in some cases suffer slight injury even with anti-trap protection provided. The irregularity of the displacement movement is likewise accompanied by undesirable audible and visual characteristics.
Moreover, the data recognition algorithm cannot reliably reflect all the operating ranges and physical conditions, such as changes resulting from mechanical aging, and requires complex adjustments for the simulation of same.